In the fanfiction, Kakashi might fail because of external pressures. For example, he could be given other important missions that take up a lot of his time and energy. So, he can't dedicate enough time to Sasuke and Sakura's development. Also, he could be facing political pressure within the village that restricts his teaching methods or the resources he can use for their training. This would lead to him not being able to meet their needs and ultimately failing them.
Perhaps Kakashi fails in this fanfiction because he misjudges Sasuke and Sakura's individual needs. Sasuke might need more emotional support due to his dark past, but Kakashi focuses mainly on his physical and ninja skills training. For Sakura, she may need more advanced medical ninjutsu training, but Kakashi overlooks this. Additionally, Kakashi could be in a state of burnout. He has been a ninja for a long time, and all the battles and losses he has experienced could make him less effective as a teacher, resulting in his failure towards Sasuke and Sakura.
In this fanfiction scenario, Kakashi's failure might cause Sasuke to be more reckless. Sasuke, feeling let down, could go off on his own more often. Sakura, who might have relied on Kakashi to keep Sasuke in check, would then be left to try and deal with Sasuke on her own. This could put a lot of stress on their relationship. She might be more desperate to bring Sasuke back to the right path, but Sasuke could see her as being too pushy, all because of Kakashi's initial failure.
One reason could be a divergence in their characters' values. Maybe in the fanfiction, Kakashi's actions or decisions go against what Sakura believes in. For example, if he's shown as being too lenient with enemies when Sakura is all about strict justice.
One possible reason could be that Kakashi might be haunted by his past experiences. He has lost many comrades, and perhaps he's afraid that getting too close to Naruto and training him would put Naruto in danger as well. He might think that Naruto, being the Jinchuriki of the Nine - Tails, already has a target on his back, and training him would only make that target bigger.
Perhaps in this fanfiction's universe, Naruto's goals and ideals are completely different from Kakashi and Sakura's. Naruto may be aiming for a more radical change in the ninja world, while Kakashi and Sakura are more conservative. Their differences could lead to Naruto developing a sense of hatred towards them as they constantly oppose his ideas.
One possible plotline could be a story set in an alternate universe where Sakura is pregnant. Kakashi could be her mentor, guiding her through this new phase of life while also dealing with Sasuke's return to the village. Sasuke might initially be hesitant about the pregnancy but then gradually grow to accept and support Sakura.
One possible reason could be that Naruto might feel Kakashi is too strict with his training. Maybe Kakashi's methods are too harsh for Naruto, making him frustrated and gradually developing dislike.
One possible reason could be Kakashi's reputation as a strict and powerful teacher. Naruto might be in awe of his skills and fear making mistakes in front of him.
One possible reason could be that Naruto might be trying to prove his independence. He has grown and wants to show that he can handle things on his own without always relying on Kakashi's guidance.
There are multiple possible reasons. Sakura's skills in both medical ninjutsu and combat are quite remarkable. Her ability to keep up with the boys in terms of strength and strategy might be appealing to Kakashi. Additionally, her emotional intelligence, like her ability to understand and comfort others, could also be a factor that makes Kakashi seem to favor her in fanfiction.
One possible scenario could be Kakashi fails to train Naruto properly. Maybe he misses some crucial aspects of Naruto's development, like not focusing enough on his chakra control early on. Another could be that Kakashi fails in a mission where Naruto is involved, perhaps due to miscommunication or underestimating the enemy.